1. Field of the Invention
The present invention relates to a dither method, and more particularly, to a dither method for converting color display information into pixel information of a flat panel display, such as a liquid crystal display device. In addition, the present invention relates to a circuit for a dither method that uses fewer color levels to display more colors.
2. Description of the Related Art
A cathode ray tube (CRT) is the most general display device. A CRT reproduces color display information by using electron guns for a red color, a green color and a blue color. As the display screen size becomes larger, the thickness of the CRT also must be thicker, since a certain distance between the electron guns and the screen of the CRT is required in order for the CRT device to reproduce the images. As a result, the CRT is no longer a proper display device when the small private display device, such as TV, is replaced with the large public device, such as a beam projector.
In recent years, many flat display devices have been developed as alternatives to the CRT. Among them, a liquid crystal display (LCD) device has become more popular than others. Conventionally, an LCD comprises, as shown in FIG. 1, a controller IC 10, a scan line driver IC 12, a signal line driver IC 11 and an array of thin film transistors (TFT) 11. A plurality of scan lines 16 are connected to the scan line driver IC 12. A plurality of signal lines 15 are connected to the signal line driver IC 11. At the intersection area of the scan lines 16 and the signal lines 15, the thin film transistors 13 connecting with pixel electrodes 14 are formed in an array. Gate electrodes of the thin film transistors 13 are connected to the scan lines 16, source electrodes thereof are connected to the signal lines 15, and drain electrodes thereof are connected to the pixel electrodes 14. When a certain voltage is applied to the gate electrodes of the TFT 13, the source electrodes of the TFT 13 and the drain electrodes are electrically connected. If there is no voltage at the gate electrodes, the source and the drain electrodes are electrically isolated.
The conventional method for reproducing the image information in the LCD device will now be described. The image information is converted into a signal voltage at the controller IC 10 and the signal voltage is held at the signal line driver IC 11. The signal line driver IC 11 sends the signal voltage to the signal lines 15 according to the scan signal. For example, when the scan line driver IC 12 sends the scan voltage to the first scan line 16 according to a predetermined frequency signal, the TFTs 13 connected to the first scan line 16 are turned on and the signal voltages corresponding to the first line of the image information are applied to the first line of the pixel electrodes 14 in the array. When the scan line driver IC 12 sends the scan voltage to the second scan line 16, the signal voltages corresponding to the second line of the image information from the signal line driver IC 11 are applied to the second line of the pixel electrodes 14. Similarly, the signal voltages corresponding to other lines of the image information are applied to the remaining pixel electrodes 14 in the array. As a result, the image information is reproduced at the screen of the LCD.
In order to reproduce the image information having colors, the image information is divided into color information comprising red (R), green (G) and blue (B) colors. These color elements (R, G and B) are joined together at each pixel of the LCD screen.
The conventional method for reproducing the color information in an LCD device will now be described. FIG. 2 shows the structure of a conventional controller IC in the color LCD devices. The conventional controller IC comprises a read only memory (ROM) table 21 having color data bits that are sent to the signal lines according to a horizontal sync signal (H.sub.s) and a vertical sync signal (V.sub.s). In addition, a latch 22 gets an input image data according to a clock signal (CK) and sends an address signal to the ROM table 21. A frame rate controller (FRC) 20 sends a signal for determining a dot position and a frame page of the color data from the ROM table 21.
As shown in FIG. 2, the input color data including L bits from a video processing unit, such as a VGA card, are sent to the latch 22 according to the clock signal (CK). At the latch 22, the input color data are translated to address bits representing the address of the color data in the ROM table 21. The FRC 20 determines the scan line to which the dot belongs according to the horizontal sync signal (H.sub.s), and determines the frame page of the color data according to the vertical sync signal (V.sub.s). That is, the input color data is used to generate the address data of the ROM table 21 having the output color data. The output color data from the ROM table 21 is applied to the signal line driver IC. The output color data determines the voltage level for driving the liquid crystal. As a result, the color image is reproduced at the LCD screen according to the driving voltage level of the liquid crystal.
Here, the number of colors is determined by the bit number of the output color data. If the bit number L is 3, then the color dots, R, G and B, have 3 bit color level. As a result, the color number of one pixel is 2.sup.9. That is, 512 colors can be reproduced at the same time. The so-called "true color" is that the color dots, R, G and B, have the 8 bit color levels, so that the number of colors in one pixel is 2.sup.24 (=16,777,216). The display for the true color can reproduce the 16.7 Mega colors at the same time.
In the controller IC, the number of bits of the input color data is 8 bits when the input color data is the true color. However, the output color data is not 8 bits. Because the driver IC for 8 bits is very expensive, the total price of an LCD is too expensive if driver IC for 8 bits is used. Generally, the price of the driver IC for 3 bits or 6 bits is from $5 to $9, and that of the 8 bits is from $25 to $40. Furthermore, if the output data bus line is 8 bits, the method for manufacturing the LCD panel is more complicated than using data bus line with less than 8 bits. Consequently, there are many research and development efforts for reproducing the true color using less than 8 bits.
In addition, in the conventional controller IC, a ROM table is used for reproducing the color information. The ROM table is also very expensive. Even though the output color data is to be 6 bits, the frames for reproducing the true color must have different color levels. As a result, the ROM table is indispensable. This leads to the high cost for manufacturing the LCD.
Accordingly, many difficulties arise with the conventional LCD device when attempting to display colors with a very high resolution. In order to increase the number of displaying colors, the data bits for representing the color must be increased and the lining structure for the color data lines is, in turn, more complicated. As a result of having more color data bits, it becomes more expensive to manufacture the LCD.